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Related Catalytic Reactions

Cathode HER Electrocatalysts

Introduction

The use of renewable resources is critical to release mankind from the current energy crisis. Being clean, renewable, and high in energy content (120 MJ kg-1), hydrogen is one of the most promising energy carriers and plays a key role in hydrogen economy,14 However, the majority of the hydrogen for current usage is from fossil fuel reforming, and the emission of CO, into the atmosphere is inevitable.15 The production of hydrogen from water using green and renewable power is a promising alternative. Electrochemical energy conversion systems such as a chlor-alkali electrolyzer,16 water-alkali electrolyzer,17 metal-O, batteries,18-22 solar water splitting devices,23*24 artificial leaves,25 and proton exchange membrane electrolyzer26-29 have been developed to produce clean hydrogen energy. However, during the processes, polarization occurs on the surface of electrodes, resulting in increased electrolysis voltage and high electricity consumption.30 Additionally, the scarcity, intolerable cost, and the limited stability of the currently most efficient platinum (Pt)-group catalysts prevent commercialization of these technologies.31 Hence, the development of low-cost, highly efficient, and stable HER electrocatalysts for large-scale and eco-friendly production of hydrogen is urgent and significant.

The equations and pathways of a HER are summarized in Table 2.1.32 The reaction could be divided into two main parts: the formation of H* (Volmer step) and the generation of H, (Heyrovsky step or Tafel step). Unlike the Volmer reaction in acidic electrolytes, that in alkaline media requires an additional step of water predissociation, which would probably introduce an energy barrier that may affect the reaction rate, w'ith the formation of H' on the catalyst surface, there are two possibilities. One possibility is the Heyrovsky reaction, in which the adsorbed hydrogen atom combines with an electron transferred from the electrode surface and a proton from the electrolyte to form a hydrogen molecule; the other is the Tafel reaction, in

TABLE 2.1

Overall Reaction Pathways for the HER in Acidic and Alkaline Solutions

Overall Reaction (condition)

Reaction Pathway

2H* + 2e' -> H2 (Acidic solution)

НЮ* + e + *-> H* + H20 (Volmer)

HiO* + e + H" -» H2 + H20 (Heyrovsky) H' + H* -*H2(Tafel)

2H,0 + 2e- -> H,+20H- (Alkaline solution)

H20+e->H' + OH(Volmer)

H20+ e + H* -> H2 + OH-(Heyrovsky) or H‘ + H' -> H2(Tafel)

which two (adjacent) adsorbed hydrogen atoms combine to form a hydrogen molecule. The possible rate controlling step(s) can be determined simply by the Tafel slope value of the polarization curves. Due to strong metal-OH.ld interactions and the high water dissociation energy barriers in alkaline solutions,33 the HER process in an alkaline medium is more sluggish than that in an acidic electrolyte. Nonetheless, it is still of great practical significance to achieve effective water electrolysis in an alkaline liquid electrolyte because several industrial processes for large-scale and low-cost hydrogen production are conducted under alkaline conditions.

 
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